Declaring and Using Scanner in Java: A Comprehensive Guide for Beginners

When you're working on a Java project and need to obtain input, the Scanner class from the java.util package is an essential tool. This guide will walk you through how to declare a Scanner, read different types of input, and handle file scanning with best practices such as the try-with-resources block. By the end, you'll be able to effectively use the Scanner class in your Java programs.

Declaring a Scanner Object

The Scanner class is initialized by passing in the standard input stream, typically represented by the object.

Scanner sc  new Scanner();

This Scanner object can then be used to read data from the keyboard, file input streams, or other sources.

Reading Input from the Keyboard

Here's a simple example to demonstrate how to create a Scanner object and read data from the keyboard:

import ;public class ScannerExample {    public static void main(String[] args) {        Scanner in  new Scanner();        String name  (); // Read a line of text        int age  (); // Read an integer        // Closing the scanner is unnecessary when using try-with-resources        (); // Not needed with try-with-resources    }}

Reading Strings and Line-by-Line Input

Below is a more detailed example to read a string and line-by-line input from the keyboard:

import ;public class ScannerExample {    public static void main(String[] args) {        Scanner in  new Scanner();        // Read a string        String name  ();        ("Name is: "   name);        // Read a series of integers        int num  ();        ("Number: "   num);        // Reading a line of text        String line  ();        ("Line of text: "   line);        // Cleardown work with the scanner        ();    }}

Reading Input from a File

When reading input from a file, using the try-with-resources block is a good practice. This ensures that the file resources are automatically closed after the operations are completed.

Path filePath Path.of(sourceTxtFileLocation) ListString lines new ArrayListString(); String wordSeparatorRegex "[s, ] "; ```java try (Scanner scanner new Scanner(filePath)) { while (scanner.hasNextLine()) { ListString words (().split(wordSeparatorRegex)); (System.out::println); } } ```

Extending the Scanner Class

While the default Scanner class is great for most use cases, you might want to extend it to suit more specific needs. Here's a basic example of extending the Scanner class:

import ;class CustomScanner extends Scanner {    public CustomScanner(InputStream stream) {        super(stream);    }    public int readInt() {        return nextInt();    }    public String readLine() {        return nextLine();    }}public class CustomScannerExample {    public static void main(String[] args) {        String sourceFile  "path/to/source-file.txt";        try (InputStream is  new FileInputStream(sourceFile);             Reader r  new InputStreamReader(is, StandardCharsets.UTF_8);             BufferedReader br  new BufferedReader(r)) {            CustomScanner scanner  new CustomScanner(br);            // Read data line-by-line            String line;            while ((line  ()) ! null) {                (line);            }        } catch (Exception e) {            ();        }    }}

Core Methods of Scanner

Here are some commonly used methods of the Scanner class:

next() Returns the next token as a String. nextInt() Reads and returns the next token as an int value. nextDouble() Reads and returns the next token as a double value. nextLine() Returns the next line of text. nextChar() Returns the next character as a String. charAt() Returns the char at the specified index in the next token.

For example:

Scanner scanner  new Scanner();String line  (); // Read a linechar firstChar  ().charAt(0); // Read the first character of the next token

Best Practices with the try-with-resources Block

Using the try-with-resources block is highly recommended when dealing with file input streams as it ensures that the resources are properly closed after the block is executed:

try (InputStream is  new FileInputStream(sourceFile);     Reader r  new InputStreamReader(is, StandardCharsets.UTF_8);     BufferedReader br  new BufferedReader(r)) {    // Process the file} catch (IOException e) {    // Handle exceptions}

Furthermore, if you need to handle streams or files, using InputStreamReader and BufferedReader in combination with Scanner is a good practice.

Conclusion

Reading input in Java is crucial for many applications, and Scanner is a powerful tool for achieving this. By understanding how to properly declare a Scanner, using it for various input types, and employing best practices like the try-with-resources block, you can handle input effectively in your Java projects.